The present invention relates to a soft-start circuit for a switching power source, and particularly to a soft-start circuit suitable for being implemented in a power source LSI chip.
The soft-start circuit for the switching power source is a circuit for preventing occurrence of an overshoot (a rush current flowing through a main SW) at starting the switching power source by gradually opening a PWM pulse width from a minimum pulse width to slowly increasing a DC converted output of the power source. As known methods of performing soft start, there are methods of performing soft start using an exponential function voltage of a CR circuit or using an integrated voltage of a capacitor are disclosed in Japanese Patent Application Laid-Open No.8-317637, Japanese Patent Application Laid-Open No.9-233806, U.S. Pat. No. 5,675,485; and a method of generating a PWM signal using a digital circuit structure according to an instruction of a microcomputer is disclosed in Japanese Patent Application Laid-Open No.9-74737; and a method of generating a ramp voltage according to an instruction data of a microcomputer through a D/A converter is disclosed in Japanese Patent Application Laid-Open No.2-7115.
In order solve the above problem, in the first method, an external CR circuit or a circuit composed of a constant current source and an external capacitor is added to the switching power source, and the soft-start time period is determined by a time constant of the CR circuit or an integrating time of the capacitor. There is a problem that the capacitor can not be implemented in the power source LSI because a large capacitor of several tens nF to several xcexcF is necessary to obtain the soft-start time period of several ms to several tens ms.
On the other hand, since the second or the third method uses a mainly digital circuit structure suitable for LSI, the digital circuit can be implemented in the LSI. However, because the digital circuit is large in circuit scale and soft-started (or initialized) by microcomputer control, it is necessary to start up a power source of the microcomputer before starting the controlled circuit (the power source circuit). Therefore, the second or the third method has a problem in that the method does not fit to general-purpose switching power sources which do not require microcomputer control.
An object of the present invention is to materialize a soft-start circuit of a switching power source, and particularly to materialize a soft-start circuit without using any external capacitor in order to eliminate external parts. Another object of the present invention is to materialize a power source IC small in scale of the soft-start circuit and suitable for a general-purpose switching power source which does not require microcomputer control.
A feature of a soft-start circuit of a switching power source in accordance with the present invention is that in the switching power source comprising a triangular wave generating circuit and an error amplifier and a PWM comparator, in normal time PWM pulses being obtained by comparing an output amplitude of triangular wave of the triangular wave generating circuit with an output voltage of the error amplifier as a reference voltage using the PWM comparator, the soft-start circuit of the switching power source comprises a soft-start reference value setting part composed of a group of resistance networks and a group of switches using the same means as an upper-and-lower limit setting part, composed of networks and switches, for setting an upper and a lower limits of the amplitude of triangular wave of the triangular generating circuit; and a counting circuit for counting cycles of the triangular wave of the triangular wave generating circuit to obtain a plurality of arbitrary soft-start time periods in order to switch the group of switches.
Another feature of a soft-start circuit of a switching power source in accordance with the present invention is constructed in such that a plurality of comparators are used instead of the counting circuit, and each of a plurality of reference voltages extracted from arbitrary voltage dividing points of an output voltage of a reference voltage generating circuit is individually added to one input terminal of each of the plurality of comparators, and a voltage corresponding to a DC converted output of the power source is input to the other input terminal of each of all the comparators.
In order to generate the plurality of reference voltages in the soft-start reference value setting part at starting the power source, cycles of the triangular wave of the triangular wave generating circuit are counted using the counting circuit, and the group of switches described above are sequentially switched every time when the calculated time reaches to each desired time. By doing so, the width of PWM pulse of the PWM comparator can be gradually increased, and therefore, the soft-start circuit having a simple circuit structure can be implemented in an LSI.
On the other hand, in the case where the plurality of comparators are used instead of the counting circuit described above, the group of switches described above are sequentially switched in decreasing order of difference between the reference voltage and the DC converted output voltage of the power source in outputs of the plurality of comparators. By doing so, the width of PWM pulse of the PWM comparator described above is gradually increased from a minimum width toward a wider width, and similarly, the soft-start circuit having a simple circuit structure can be implemented in an LSI.
The above-mentioned features and the other features of the present invention will be described below.